Computers using memory such as dynamic random access memory (DRAM) are well known in the art. Personal computers such as IBM AT and compatibles, use such DRAM memories for a variety of functions. Development in the personal computer (PC) industry has been the widespread acceptance of single inline memory modules (SIMMS). Such SIMMS are used to lower the cost of memory and many higher density memory subsystems can be designed using SIMMS. Since the SIMMS are relatively small modules, they are used extensively in the computer industry to allow for the proper operation. The DRAM chips themselves are very reliable. Therefore, errors due to DRAM are less common than when PCs were first introduced. At the same time cost of the personal computers has dropped dramatically and profit margins have been squeezed by intense competition. Typically in known computer systems a particular type of SIMM module is utilized therewithin. That SIMM module requires a particular addressing scheme for adequate operation. Hence, the addressing scheme is oftentimes limited to a particular type of DRAM, as there have become more and more different types of DRAMs. As more and different types of DRAMs come into existence with different types of addressing schemes, it becomes more difficult to design these different modules into a computer. This difficulty occurs because the memory controller used to control the SIMM module, or the memory within the computer, must generally be adaptable to driving one type of addressing scheme. To make a more complicated and complex memory controller would require additional expense to the personal computer. Many of the larger memories, that is 2 Mb DRAMs, are not compatible with existing personal computers because their addressing schemes are very different than the existing addressing scheme.
For example, in a typical 1 Mb DRAM, for example Part No. MT4C4001J manufactured by Micron Technology, Inc., the Row Address Strobe (RAS) cycle requires 10 lines and on the Column Address Strobe (CAS) cycle requires 10 lines. Hence when two banks of these 1 Mb DRAMS are utilized, this addressing scheme required above works adequately for operation thereof. However, as many of the larger DRAMs such as 2 MB DRAMs, do not utilize this addressing scheme.
For example, a 2 Mb DRAM for example Part No. IBM0117800AT3 manufactured by International Business Machines Corporation has an addressing scheme which requires 11 bits for the RAS cycle and 10 bits for the CAS cycle. Therefore, to substitute the addressing scheme described above, 1 bank of the 2 Mb DRAM with 11/10 RAS/CAS for the 2 banks of 1 Mb DRAM with 10/10 RAS/CAS would allow half of the 2 Mb DRAM bank to be accessible due to the extra bit necessary for the RAS generation.
Accordingly, many SIMM module manufacturers continue to use the smaller 1 Mb DRAMs even though they are more expensive than the 2 Mb DRAMs and take up more space in the computer system. They are easily implemented in a typical personal computer. Accordingly, the costs and size considerations have largely been addressed by existing systems. What is desired therefore is a system for allowing for different types of memory modules with different addressing schemes to be substituted for existing memory modules, so as to allow for increased memory capacity with resultant lower cost associated with the larger memory units. The system that is developed should be cost effective, should not add undue complexity to the existing circuitry, and should also be compatible with existing circuitry. The present invention addresses such a need.